Secondary batteries such as lithium ion batteries are widely used in portable digital devices such as mobile phones, notebook computers, digital cameras, digital video cameras and portable game machines. In recent years, such secondary batteries have been increasingly used as power sources for hybrid cars, electric cars, plug-in hybrid cars and so on in automobile applications.
A lithium ion battery generally has a configuration in which a secondary battery separator and an electrolyte are interposed between a positive electrode with a positive active material stacked on a positive electrode current collector and a negative electrode with a negative active material stacked on a negative electrode current collector.
As a secondary battery separator, a polyolefin porous base material is used. Characteristics required for secondary battery separators include a characteristic in which the separator contains an electrolyte solution in a porous structure so that ion movement is possible, and a characteristic in which if abnormal heat generation occurs in a lithium ion battery, the material is melted by heat to close the porous structure so that ion movement is stopped to stop power generation.
However, with an increase in capacity and power of lithium ion batteries in recent years, secondary battery separators have been required to have not only the above-mentioned characteristics, but also adhesiveness with an electrode with the separator impregnated with an electrolyte solution (wet adhesiveness) to prevent deterioration of cycle performance due to generation of gaps between the electrode and the secondary battery separator in charge-discharge repetitions. In addition, with an increase in capacity and power of lithium ion batteries in recent years, secondary battery separators have been required to have high safety. In addition, secondary battery separators have been required to have dimensional stability to prevent a short circuit resulting from contact between a positive electrode and a negative electrode, which is caused by thermal shrinkage of the secondary battery separator at a high temperature. In addition, secondary battery separators have been required to have adhesiveness between the separator and an electrode before impregnation of an electrolyte solution (dry adhesiveness) to maintain the shape of a laminate in transportation of the laminate of a positive electrode, a separator and a negative electrode, or to prevent collapse of the shape of a laminate in pressing before insertion of the wound laminate of a positive electrode, a separator and a negative electrode into a mold such as a cylindrical mold or a square mold. Further, with widespread use of secondary batteries, it has been required to reduce production costs of secondary batteries and secondary battery separators.
For these requirements, Japanese Patent Laid-open Publication No. 2004-146190 and Japanese Patent Laid-open Publication No. 2012-221741 propose a secondary battery separator in which a porous layer mainly composed of a polyvinylidene fluoride resin having wet adhesiveness is stacked on a porous base material composed of a polyolefin so that adhesiveness with an electrode is improved. Japanese Patent Publication No. 5355823 proposes a secondary battery separator in which a porous layer mainly composed of particles formed of a polyvinylidene fluoride resin having wet adhesiveness is stacked on a porous base material composed of a polyolefin so that wet adhesiveness with an electrode is improved. In addition, International Publication No. WO 2013/133074 proposes a secondary battery separator in which a porous layer composed of particles formed of a polyvinylidene fluoride resin having adhesiveness and inorganic particles is stacked on a porous base material composed of a polyolefin so that wet adhesiveness with an electrode and dimensional stability are improved.
However, in Japanese Patent Laid-open Publication No. 2004-146190 and Japanese Patent Laid-open Publication No. 2012-221741, it is proposed as a production method that a release film or a porous base material is coated with a fluororesin dissolved in an organic solvent, and immersed in a coagulation tank to form a porous layer, and that production method improves the wet adhesiveness of a secondary battery separator with an electrode, but is a high-cost production method, and thus cannot satisfy the current requirement of reducing costs for the secondary battery separator. In addition, the dry adhesiveness with an electrode in a state in which the separator is not impregnated with an electrolyte solution is not sufficient. In Japanese Patent Publication No. 5355823 and International Publication No. WO 2013/133074, the molecular weight of a polyvinylidene fluoride resin to be used is not appropriate and, therefore, sufficient wet adhesiveness with an electrode cannot be obtained. In addition, the dry adhesiveness with an electrode in a state in which the separator is not impregnated with an electrolyte solution is not sufficient.
Thus, in view of the above-mentioned problems, there is a need to provide a secondary battery separator that exhibits wet adhesiveness and dry adhesiveness with an electrode and dimensional stability at a low cost.